Before air can be introduced into a cryogenic air separation process in which oxygen and nitrogen are separated from one another, CO.sub.2 present in the air at low levels (eg. 400 ppm) must be removed in order to avoid CO.sub.2 from solidifying in the air separation plant. Two methods generally used for such CO.sub.2 removal are temperature swing adsorption (TSA) and pressure swing adsorption (PSA).
In each of these techniques, a bed of adsorbent is exposed to a flow of feed air for a period to adsorb CO.sub.2 from the air. Thereafter, the flow of feed air is shut off from the adsorbent bed and the adsorbent is exposed to a flow of purge gas which strips the adsorbed CO.sub.2 from the adsorbent and regenerates it for further use. In TSA, the CO.sub.2 is driven off from the adsorbent by heating the adsorbent in the regeneration phase. In PSA, the pressure of the purge gas is lower than that of the feed gas and the change in pressure is used to remove the CO.sub.2 from the adsorbent.
Other components can be removed from the feed air by these processes, including hydrocarbons and water. These adsorption techniques can also be applied to feed gases other than air or to air to be purified for purposes other than use in an air separation plant.
The use of PSA for removing CO.sub.2 from air prior to separating air into its respective components by cryogenic air separation is described in numerous publications, eg. U.S. Pat. No. 4,249,915 and U.S. Pat. No. 4,477,264. Conventional processes employed a dual bed of alumina for water removal followed by a zeolite such as 13.times. for CO.sub.2 removal. More recently, all alumina PSA systems have been proposed, as described in U.S. Pat. No. 5,232,474. The advantages of an all alumina system include lower adsorbent cost, vessel design which does not need screens to separate the two different adsorbents and better thermal stability in the adsorption vessel during blow down and repressurization. It would be desirable however to develop adsorbents having an improved CO.sub.2 capacity so as to allow smaller bed sizes with lower capital costs and less void gas being lost during depressurization, ie. higher air recoveries.
Alumina is also used as an adsorbent in TSA and for this purpose it has been proposed to treat the alumina to form alkali metal oxides thereon to increase the adsorptive capacity of the alumina. By way of example U.S. Pat. No. 4,493,715 teaches a method for removing CO.sub.2 from olefin streams by contacting the feed gas with a regenerable, calcined adsorbent consisting of essentially from 1 to 6 wt % of an alkali metal oxide selected from the group consisting of sodium, potassium and lithium on alumina. The adsorbent was prepared by contacting alumina with an alkali metal compound which is convertible to the metal oxide on calcination.
U.S. Pat. No. 4,433,981 describes a process for removing CO.sub.2 from a gaseous stream which comprises contacting the gas stream at a temperature up to about 300.degree. C. with an adsorbent prepared by impregnation of a porous alumina with a sodium or potassium oxide. The corresponding oxide can be prepared by impregnation with a decomposable salt and calcining at a temperature of 350.degree. to 850.degree. C. Salts mentioned include alkali metal bicarbonates.
U.S. Pat. No. 3,557,025 teaches a method for producing alkalized alumina which is capable of adsorbing SO.sub.2. The adsorbent is prepared by selectively calcining the alumina, and contacting with an alkali or ammonium bicarbonate salt to form at least 30% by weight alkalized alumina having the empirical formula of MAI(OH).sub.2 CO.sub.3.
U.S. Pat. No. 3,865,924 describes the use of a finely ground mixture of potassium carbonate and alumina as an absorbent for CO.sub.2, which reacts with the carbonate and water to form bicarbonate. The absorbent mixture is regenerated by mild heating, eg. at 93.degree. C. (200.degree. F.). The presence of stoichiometric quantities of water is essential and the alumina appears to be regarded as essentially a mere carrier for the potassium carbonate. Other carbonates may be used.
U.S. Pat. No. 5,232,474 discloses a PSA process using alumina in 70-100% of the bed volume to remove water and CO.sub.2 from air. Preference is expressed for alumina containing up to 10 wt. % silica as opposed to the generality of aluminas which typically contain only about 1 % silica. Silica is an acidic material and the use of basic compounds to increase CO.sub.2 capacity as proposed herein is therefore contrary to the teaching of this document.
Those skilled in the art continue to search for adsorbents suitable for use in PSA processes which provide improved CO.sub.2 adsorption capacity and which can be regenerated under PSA operating conditions. Moreover, those skilled in the art are searching for improved CO.sub.2 adsorbents which can be used in PSA processes which are integrated with other processes wherein adsorption of CO.sub.2 onto the adsorbent and regeneration of the adsorbent are accomplished at temperatures often substantially higher than ambient temperature.